This invention relates to a method of manufacturing semiconductor devices and apparatus for the method and, in particular, to a method of etching a layer on a wafer.
Recently, with an increasing degree of integration of a semiconductor device, reduction of the number of particles has become an important issue.
To decrease the particles, a vapour-phase cleaning method has been investigated, in particular, vapour-phase hydrogen fluoride (HF) treatment is investigated as an effective method to remove native oxide and obtain a clean surface. An apparatus and method for removing native oxide layers are disclosed in PCT International Publication No. WO91/06795.
On the other hand, a new phenomenon, that an etching rate greatly depends on properties of the objective layers in vapour-phase etching, is also observed. The phenomenon is described in a paper contributed by Nobuhiro Miki and four others to the IEEE Transactions on Electron Devices, Vol. 37, No. 1 (January 1990), pages 107-115, under the title of "Gas-Phase Selective Etching of Native Oxide". The Miki et al paper reports that an etching rate of the objective layers considerably varies when a water vapour concentration in the vapour-phase hydrogen fluoride is controlled and that a change in etching rate depends on the properties, in particular, compositions of the objective layers.
More particularly, the Miki et al paper reports that reducing of the water vapour concentration in the vapour phase to 0.1 ppm or less enables selective removal of objective layers consisting of phospho-silicate glass (PSG). The process has the advantage that the objective layers can selectively be removed without etching of layers other than the objective layers.
However, the selective etching process of the Miki et al paper requires strict control of a vapour-phase hydrogen fluoride concentration and of the water vapour concentration. Reducing the water vapour concentration to less than a few ppm as described in the Miki et al paper is very difficult and costly for the following reasons. In order to reduce the water vapour concentration, the concentration of impurities in the vapour-phase hydrogen fluoride must be extremely reduced. Furthermore, an etching chamber for the selective etching process must have a structure which avoids mixing of the surrounding atmosphere and the like into the etching chamber and adherence of water to the etching chamber. In addition, etching the objective layers with vapour-phase hydrogen fluoride produces water as a reaction product. Therefore, the water vapour concentration undesiredly increases to adversely affect stable progress of the selective etching process.